Lightweight ES5 class like object definition

class_like_obj.js

var Person = {
  init: function() {
    this.first_name = '';
    this.last_name = '';
    return this;
  },
  getFullName: function() {
    return this.first_name + ' ' + this.last_name;
  },
};

p = Person.init()
// {init: ƒ, getFullName: ƒ, first_name: "", last_name: ""}
p.first_name = 'Yuki';
// "Yuki"
p.last_name = 'Yokoyama';
// "Yokoyama"
p.getFullName()
// "Yuki Yokoyama"

Oh...this is not correct way...then I got this error.

p instanceof Person
VM200:1 Uncaught TypeError: Right-hand side of 'instanceof' is not callable
    at <anonymous>:1:3

function Product(first_name, last_name) {
  this.first_name = first_name;
  this.first_name = last_name;
}
pr = new Product('Yuuki', 'Yokoyama')
pr instanceof Product
true

Resource (ja)

• JavaScriptとオブジェクト指向プログラミング | POSTD
• 中上級者になるためのJavaScript【知識編】 - Qiita
• JavaScript (ES5) でクラスを実現するための基本 - Qiita